The present invention is directed to a golf ball core component that includes a central portion and a relatively soft skin portion that surrounds the central portion. Various preferred embodiment golf balls are described that utilize such a core component and further include one or more interior wound layers and/or multi-layer covers.
Sound and feel are two qualities of golf balls which are typically judged subjectively. For the most part, however, soft sound (xe2x80x9cclickxe2x80x9d) and soft feel (i.e., low vibrations) are golf ball qualities desired by many golfers. If a soft feeling ball is mis-hit, the adverse sting felt in a golfer""s hands is not as great as if a harder feeling ball is hit improperly. A soft sounding ball has a soft low pitch when hit with any club, but particularly off a putter.
One way to achieve a soft sound and feel is to provide a softened layer between the core and the cover. The prior art teaches development of a three piece ball or a multi-layer cover. However, adding additional layers is costly and can sometimes lead to non-uniform layers.
U.S. Pat. No. 4,650,193 to Molitor et al. describes a two-piece golf ball comprising a core and a cover. The core has a central portion of a cross-linked, hard, resilient material and a soft, deformable outer layer. The cover is a conventional cover. The soft, deformable outer layer of the core is integral with the core. It is formed by treating a slug of an elastomeric material with a cure altering agent, namely elemental powdered sulfur, so that a thin layer of sulfur coats the surface. The sulfur-coated slug is then cured in a molding cavity at temperatures greater than 290xc2x0 F., e.g., 325xc2x0 F., for 10-20 minutes, depending on core temperature.
According to the ""193 patent, sulfur on the surface of the slug penetrates a surface layer to a depth of about {fraction (1/16)} inch during curing. Wherever the core is exposed to sulfur, the conventional peroxide cure is altered, resulting in an amorphous soft outer layer. The portion of the core that is not touched by the sulfur cures normally and becomes relatively crystalline. The final result is a spherical core having a hardness gradient in its surface layers.
The present inventors seek to achieve somewhat of a similar effect using methods which do not require the addition of elemental sulfur to modify and soften the core surface such that the cure on the core surface is retarded. At the same time, the inventors seek to maintain the parameters of resilience and hardness of the finished ball at desired levels.
Resilience is determined by the coefficient of restitution (C.O.R.), the constant xe2x80x9cexe2x80x9d, which is the ratio of the relative velocity of two elastic spheres after direct impact to that before impact, or more generally, the ratio of the outgoing velocity to incoming velocity of a rebounding ball. As a result, the coefficient of restitution (i.e., xe2x80x9cexe2x80x9d) can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision. Hardness is determined as the deformation (i.e., Riehle compression) of the ball under a fixed load of 200 pounds applied across the ball""s diameter (i.e., the lower the compression value, the harder the material).
Resilience (C.O.R.), along with additional factors such as clubhead speed, angle of trajectory, and ball configuration (i.e., dimple pattern), generally determines the distance a ball will travel when hit. Since clubhead speed and the angle of trajectory are not factors easily controllable, particularly by golf ball manufacturers, the factors of concern among manufacturers are the coefficient of restitution (C.O.R.) and the surface configuration of the ball.
In this regard, the coefficient of restitution of a golf ball is generally measured by propelling a ball at a given speed against a hard surface and measuring the ball""s incoming and outgoing velocity electronically. The coefficient of restitution must be carefully controlled in all commercial golf balls in order for the ball to be within the specifications regulated by the United States Golfers Association (U.S.G.A.).
Along this line, the U.S.G.A. standards indicate that a xe2x80x9cregulationxe2x80x9d ball cannot have an initial velocity (i.e., the speed off the club) exceeding 255 feet per second (250 feet per second with a 2% tolerance). Since the coefficient of restitution of a ball is related to the ball""s initial velocity (i.e., as the C.O.R. of a ball is increased, the ball""s initial velocity will also increase), it is highly desirable to produce a ball having a sufficiently high coefficient of restitution to closely approach the U.S.G.A. limit on initial velocity, while having an ample degree of hardness (i.e., impact resistance) to produce enhanced durability.
The coefficient of restitution (C.O.R.) in solid core balls is a function of the composition of the molded core and of the cover. In balls containing a wound core (i.e., balls comprising a liquid or solid center, elastic windings, and a cover), the coefficient of restitution is a function of not only the composition of the center and cover, but also the composition and tension of the elastomeric windings.
An object of this invention is to develop a method for improving the sound and feel of a golf ball without adversely affecting the resilience or coefficient of restitution of the ball. The method does not require the addition of sulfur based chemicals to an uncured slug, in order to minimize the steps involved. In addition, the softer golf ball produces the playability characteristics desired by the more skilled golfer. It also enhances durability characteristics, as the outer skin is flexible and resists crack propagation.
These and other objects and features of the invention will be apparent from the following summary and description of the invention and from the claims.
The present invention provides, in one aspect, a golf ball comprising a core component having a central portion with a Shore C hardness of from about 50 to about 90, and an integral skin portion disposed on the central portion, the skin having a Shore C hardness of from about 30 to about 70. The golf ball further includes a cover component disposed on the core component and generally surrounding the core component. The cover component may consist of single or multiple layers.
In another aspect, the present invention provides a golf ball comprising a core component having a central portion and a skin portion disposed about the central portion. The central portion is harder than the skin portion, and both central and skin portions are formed in-situ from the same material or different material. The golf ball may further include a wound layer and a cover component disposed about the wound layer. The cover component may consist of one or more layers.
In yet another aspect, the present invention provides a golf ball comprising a core component having a central portion and a skin portion disposed about the central portion. The hardness of the central portion is at least 20 Shore C units greater than the hardness of the skin portion. The ball further comprises a wound layer disposed about the core component, and a cover component surrounding the wound layer. The cover component may consist of single or multiple layers.
In a further aspect, the present invention provides a method for producing a golf ball core component having a central portion and a skin portion disposed on the central portion, such that the skin portion is softer than the central portion. The method comprises depositing a slug of polymeric material capable of undergoing an exothermic curing reaction, in a molding chamber. The slug is then subjected to curing conditions to cause the temperature within the interior of the molding chamber to increase. The molding chamber is cooled to thereby cause the temperature at the surface of the slug to be less than the temperature within the interior of the slug. This results in a golf ball core having a central portion and a softer skin portion. The core is then enclosed by one or more cover layers. Optionally, a wound layer can be disposed on the core under the cover layer(s). The golf ball produced by this method is also included in the present invention.
In another aspect, the present invention provides a method for producing a golf ball core component having a central portion and a skin portion disposed on the central portion such that the skin portion is softer than the central portion. In this aspect, the method includes exposing a slug of polymeric material to water such that the slug absorbs water. The slug is then deposited within a molding chamber of a molding apparatus and the polymeric material is cured. As a result of the water absorbed about the surface of the polymeric slug, a golf ball core component having the central portion and a softer skin portion surrounding the central portion is produced. The core is then encapsulated by a wound layer and/or one or more cover layers.
In another aspect, the present invention provides a method for producing a golf ball core component having a central portion and a skin portion surrounding the central portion. The method involves depositing a cross-linking retardant agent on the surface of a polymeric slug. The slug is placed within a molding chamber and the slug is then cured. The resulting golf ball core component includes a relatively soft skin that surrounds a harder central portion. The core is subsequently enclosed by a wound thread layer and/or one or more cover layers.
These and other advantages of the invention will become apparent from the detailed description provided below.